No abstract

The class III NAD+ dependent deacetylases-sirtuins (SIRTs) link transcriptional regulation to DNA damage response and reactive oxygen species generation thereby modulating a wide range of cellular signaling pathways. Here, the contribution of SIRT1, SIRT3, and SIRT5 in the regulation of cellular fate through autophagy was investigated under diverse types of stress. The effects of sirtuins' silencing on cell survival and autophagy was followed in human osteosarcoma and mesothelioma cells exposed to DNA damage and oxidative stress. Our results suggest that the mitochondrial sirtuins SIRT3 and 5 are pro-proliferative under certain cellular stress conditions and this effect correlates with their role as positive regulators of autophagy. SIRT1 has more complex role which is cell type specific and can affect autophagy in both positive and negative ways. The mitochondrial sirtuins (SIRT3 and SIRT5) affect both early and late stages of autophagy, whereas SIRT1 acts mostly at later stages of the autophagic process. Investigation of potential crosstalk between SIRT1, SIRT3, and SIRT5 revealed several feedback loops and a significant role of SIRT5 in regulating SIRT3 and SIRT1. Results presented here support the notion that sirtuin family members play important as well as differential roles in the regulation of autophagy in osteosarcoma vs. mesothelioma cells exposed to DNA damage and oxidative stress, and this can be exploited in increasing the response of cancer cells to chemotherapy.

Acute lymphoblastic leukaemia (ALL) is the most frequent childhood cancer and, although it is highly treatable, resistance to therapy, toxicity and side effects remain challenging. The synthetic glucocorticoid (GC) dexamethasone (dex) is commonly used to treat ALL, the main drawback of which is the development of resistance to this treatment. The aim of the present study was to investigate potential molecular circuits mediating resistance and sensitivity to GC-induced apoptosis in ALL. The leukaemia cell lines CEM-C7-14, CEM-C1-15 and MOLT4 treated with chloroquine (CLQ), thapsigargin (TG) and rotenone (ROT) were used to explore the roles of autophagy, endoplasmic reticulum (ER) stress/unfolded protein response (UPR) and reactive oxygen species (ROS) generation in the response to GC treatment. ROS levels were associated with increased cell death and mitochondrial membrane potential in rotenone-treated CEM cells. Autophagy inhibition by CLQ exhibited the strongest cytotoxic effect in GC-resistant leukaemia. Autophagy may act as a pro-survival mechanism in GC-resistant leukaemia since increasing trends in beclin-1 and microtubule-associated protein 1 light chain 3α levels were detected in CEM-C1-15 and MOLT4 cells treated with dex, whereas decreasing trends in these autophagy markers were observed in CEM-C7-14 cells. The intracellular protein levels of the ER stress markers glucose-regulated protein (GRP)78 and GRP94 were stimulated by dex only in the GC-sensitive cells, suggesting a role of these chaperones in the GC-mediated ALL cell death. Increased cell surface levels of GRP94 were recorded in CEM-C7-14 cells treated with combination of dex with TG compared with those in cells treated with TG alone, whereas decreasing trends were observed in CEM-C1-15 cells under these conditions. Taken together, the results of the present study demonstrated that autophagy may be a pro-survival mechanism in GC-resistant leukaemia, and by modulating intracellular and surface GRP94 protein levels, dex is involved in the regulation of ER stress/UPR-dependent cell death and immune surveillance. These observations may be of clinical importance if confirmed in patients.